Seismic events, such as earthquakes, generate energy that travels through the planet as seismic waves. These waves offer scientists the primary means of investigating the Earth’s deep interior and pinpointing the exact location of a tremor. Primary (P) waves and Secondary (S) waves are the two main forms studied by seismologists. Although they possess significant differences in motion and speed, P waves and S waves share fundamental characteristics that define their identity and utility in the study of our planet.
Shared Identity as Body Waves
Both P waves and S waves belong to the category of “body waves,” meaning they travel through the Earth’s interior, moving through the mantle and core. This distinguishes them from surface waves, which are confined to the upper layers of the crust. Both waves originate simultaneously at the earthquake’s focus, or hypocenter, the point deep underground where the rupture first occurs. The energy is released from this single source, radiating outward in all directions. Their behavior is linked to the physical conditions of the material they travel through, such as temperature, pressure, and composition.
Transmission Through Earth’s Interior
Both P waves and S waves are mechanical waves, meaning they require a physical medium—like rock, soil, or fluid—to propagate. They rely on the elastic properties of the transmitting material to transfer energy. As the wave passes, the rock is temporarily deformed and then springs back to its original shape, continuing the wave’s journey.
The speed and path of both wave types are governed by the density and rigidity of the material they encounter at depth. As P and S waves travel deeper, the increasing pressure and density generally cause the velocity of both waves to increase. This change in speed causes both waves to bend, or refract, as they pass through different layers, resulting in curved paths.
Both wave types are also subject to reflection and refraction when they strike a boundary between two layers with different properties, such as the boundary between the mantle and the core. These shared phenomena allow seismologists to use both waves to map the distinct boundaries and compositional changes within the Earth’s structure.
Common Utility in Earthquake Location
Despite their difference in arrival time, both P waves and S waves are necessary for precisely locating the source of an earthquake. P waves travel faster and are the first to be recorded by a seismograph, followed shortly by the slower S waves. The time difference between the arrival of the two waves, known as the S-P interval, is the fundamental metric used to determine the distance from the recording station to the earthquake’s epicenter.
A greater time difference between the two arrivals indicates a greater distance from the earthquake source. This distance measurement establishes a radius around the single seismic station. The process requires data from at least three different seismic stations, where the three corresponding distance circles are plotted on a map. The point where all three circles intersect indicates the precise location of the earthquake’s epicenter, a method known as triangulation.